A crystalline silicon-based solar cell using a crystalline silicon substrate has high photoelectric conversion efficiency, and is practically used as a photovoltaic power generation system. A heterojunction solar cell provided with a pn junction by depositing a non-single-crystalline silicon-based semiconductor thin-film of amorphous silicon or the like on a surface of a single-crystalline silicon substrate is excellent in conversion efficiency. In particular, a heterojunction solar cell including an intrinsic amorphous silicon thin-film between a single-crystalline silicon substrate and an amorphous silicon thin-film attracts attention as one of solar cells having the highest conversion efficiency.
In a crystalline silicon-based solar cell using a single-crystalline silicon substrate, such as a heterojunction solar cell, a surface of a silicon substrate is anisotropically etched to form a quadrangular pyramid-shaped (pyramidal) concave-convex structure so called texture, so that the surface reflectance is reduced and the optical path length of light incident to a crystalline silicon substrate is increased, and thereby increasing the photoelectric current to improve efficiency (so called optical confinement). The single-crystalline silicon substrate after formation of the texture is further subjected to other treatments for the purpose of, for example, adjusting the shape of the texture and cleaning the surface before formation of a semiconductor thin-film and a diffusion layer.
For example, Patent Document 1 discloses a method in which a texture is formed on a surface of a single-crystalline silicon substrate by anisotropic etching, and isotropic etching is then performed using an oxidizing aqueous solution such as HF/HNO3, so that recesses of the texture are rounded. Patent Document 1 suggests that when recesses of the texture are rounded, an amorphous silicon thin-film formed on the texture has a uniform thickness, so that the open circuit voltage and the fill factor of a solar cell are improved.
Patent Document 2 discloses a method in which after formation of a texture as necessary, a treatment is performed to round recesses of the texture, and a single-crystalline silicon substrate is then immersed in ozone water to form an oxide film. When the silicon substrate after formation of the oxide film is immersed in an HF aqueous solution, metals and organic substances penetrating during formation of the texture by anisotropic etching are removed together with the oxide film, so that a surface of the silicon substrate is cleaned.
Patent Document 3 indicates that since a layer of a high-concentration silicon dissolution product (gel) is formed on a surface of a silicon substrate after anisotropic etching with an alkali, it may be difficult to clean the surface by an ozone treatment. Patent Document 3 suggests that ozone water is continuously supplied while the ozone concentration of the ozone water is made constant, thereby compensating for a reduction in efficiency due to autolysis of ozone to improve surface cleanliness by the ozone treatment. Patent Document 3 (see FIG. 12) describes a method in which the ozone concentration of the ozone water is made constant by supplying an ozone gas into an ozone water bath thereby improving treatment efficiency.